Abstract
In this work, we studied the effect of alkali-activated zeolite foams modifications on properties and catalytic activity of cobalt phases in the process of catalytic decomposition of N2O. The zeolite foam supports were prepared by alkali activation of natural zeolite followed by acid leaching and ion exchange. The cobalt catalysts were synthesised by a different deposition technique (direct ion exchange (DIE) and incipient wetness impregnation (IWI) method of cobalt on zeolite foams. For comparison, catalysts on selected supports were prepared and the properties of all were compared in catalytic tests in the pellet form and as crushed catalysts to determine the effect of internal diffusion. The catalysts and supports were in detail characterized by a variety of techniques. The catalyst activity strongly depended on the structure of support and synthesis procedure of a cobalt catalyst. Ion exchange method provided active phase with higher surface areas and sites with better reducibility, both of these factors contributed to higher N2O conversions of more than 80% at 450 °C. A large influence can also be attributed to the presence of alkali metals, in particular, potassium, which resulted in a modification of electronic and acid base properties of the cobalt oxide phase on the catalyst surface. The promotional effect of potassium is better reducibility of cobalt species.
Highlights
The alkali-activated materials have properties very similar to zeolites but do not form crystalline structures
A part of the alkaline activator occurred in a form of mixed Na/K silicate, which can be seen in images from scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) analyser as a leaf-like particles (Figure 1)
The results obtained on cobalt catalysts using zeolite foam as support (AA-N-Co, AA-ion exchange (IE)-Co, AA-IE-CoMn) are comparable to the results presented on other types of catalysts, e.g., CoMnAl mixed oxide catalysts prepared from a hydrotalcite precursor are very efficient and show similar results under the same conditions, but as other authors have shown in their studies of catalysts, other parameters such as Co/Mn ratio or type and content of alkalis (Na, K, Cs) need to be sensitively optimized [11,27]
Summary
The alkali-activated materials have properties very similar to zeolites but do not form crystalline structures. Raw materials for the preparation of these materials, e.g., natural zeolites, most often occur as compact fine-grained rocks formed by sedimentary or volcanic activity, whose main component is zeolite clinoptilolite, belonging to the group of heulandite, accompanied by minerals from the group of feldspars, clays and mica. Due to their properties, natural zeolites are widely used as sorbents for water purification (Cs+ , Rb+ and NH4+ ) or gases (NH3 ) [1,2]. Macroporosity is advantageous for diffusion reasons and for structural reasons, as this material reduces weight of catalyst/sorption bed and the complexity of the equipment and its robustness requirements [6]
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